1. Field of the Invention
The present invention relates to an image forming apparatus for forming an image on a recording medium, and more particularly to an image forming apparatus suitable for a color copying machine.
2. Description of the Prior Art
In a color image reading system in a document reader of a prior art color copying machine, three color-decomposed signal data for blue(B), green(G) and red (R) are produced in one scan of the document and those signal data are temporarily stored in memories for use in color printing, or the document is scanned a plurality of times one for each of the three primary colors, B, G and R to effect the color printing.
In the former one-scan system, the signal data for the three colors are produced in one scan and no color shift during the scan takes place. But, it requires a relatively large capacity of memory and a cost is high. On the other hand, in the latter plural-scan system, the temporary memory is not required but there is a problem of ununiformity of document readout in the plurality of scans.
The ununiformity in the document scan (readout) is considered to be due to a change of a viscosity resistance in a drive unit of a document reader, a change of a sliding friction, a change of a load, an expansion of a drive wire and a vibration of the drive unit. If the ununiformity is included in the document scan, the colors on a record sheet are misaligned. Eyes of human beings are very sensible to such misalignment.